Sustainable Drainage Systems

CC2 Sustainable Drainage Systems CC2: Sustainable Drainage Systems (SuDS) 1. All new development, except householder development* must incorporate Sustainable Drainage Systems (SuDS) ensuring that all the following criteria are met: 2. a) There is no material increase in run-off rates at the site boundary. b) For greenfield sites, limit run-off rates to existing greenfield rates or lower where feasible. c) For brownfield sites, reduce run-off rates as close as practicable to greenfield run-off rates. 3. SuDS should be designed in accordance with the most recent version of the National Non- Statutory SuDS Standards (June 2025 as replaced) and the CIRIA SuDS Manual (C753 as replaced), which provide nationally recognised best practice guidance. This includes but is not limited to: a) No flooding in 1 in 30-year storm events. b) Safe containment of flooding in 1 in 100-year storm events plus climate change; and c) 10% urban creep allowance where appropriate. 4. When designing a surface water drainage scheme, consideration must be given to the drainage hierarchy outlined in national guidance. The order of preference is as follows: a) Collected for non-potable use. b) Infiltration to ground. c) Discharge to a surface water body, where applicable this must be supported by in principle third-party permission. d) To a surface water sewer, or another drainage system. e) To a combined sewer; only in exceptional circumstances will surface water connections to the combined system be permitted. Applicants will need to demonstrate consultation with the sewerage undertaker to demonstrate that there is no feasible alternative and that there will be no detriment to existing users. 5. SuDS schemes must be supported by site-specific ground investigations, including infiltration rate testing and groundwater level monitoring over the winter period (from the start of October to end of April). Where infiltration is not viable, above-ground attenuation must be used. 6. SuDS must be accompanied by a maintenance schedule and operation manual. Maintenance responsibilities and funding mechanisms must be secured via planning condition or legal agreement. 7. Within the site: a) SuDS schemes should demonstrate how site constraints have been considered in the design of the drainage strategy and how this design has been considered in the site masterplan to provide multifunctional benefits e.g. water quantity, water quality, amenity and biodiversity. b) Priority must be given to exemplar above-ground source control SuDS which mimic and reflect natural drainage processes. c) Details of the SuDS scheme future maintenance over the lifetime of the development must be included in the form of a management plan to be agreed and contributions will be required for the maintenance of the SuDS. d) Proposed hard surfacing must be permeable unless such surfacing cannot be made permeable and development proposals should include rainwater re-use and collection mechanisms such as green roofs/walls, rainwater gardens and in residential proposals water collection and recycling facilities such as a rainwater butts. e) SuDS for hard-standing areas for parking of 50 or more cars, or equivalent sized areas, will be expected to include appropriate additional treatment stages/ interceptors to ensure that any pollution risks are suitably addressed. f) SuDS required must consider the impact on food production areas and reducing the risk of damaging crops. 7. Documentation requirements for SuDS must be proportionate to the scale of development and include drainage strategy, infiltration testing and maintenance plan. *Householder development to be exempt from the above requirements must demonstrate the use of permeable surfaces and increased greening and or rainwater storage to control runoff. Where no external space exists (for example flatted development) exemptions must be justified and alternative mitigation provided. Definition of SuDS 7.530 Sustainable drainage systems (SuDS) aim to reduce the impact of development by replicating the natural processes through which rainwater is captured, stored, and transported within a development. Traditional piped drainage systems remove runoff from a site as quickly as possible, however SuDS slow flow and store runoff onsite before infiltrating into the ground or slowly releasing it offsite. When designed correctly, SuDS provide multifunctional benefits: water quantity, water quality, biodiversity and amenity. 7.531 All SuDS should be designed in accordance with the most recent National Planning Policy Framework and the Planning Practice Guidance and the non-Statutory SuDS Standards and CIRIA SuDS Manual. The National SuDS Standards (2025) were developed collaboratively by DEFRA, the Environment Agency, and industry experts. They reflect the latest understanding of climate resilience, water quality, and multifunctional SuDS design. Their use ensures that developments meet the highest standards of sustainable water management. While nonstatutory, they are considered material to the assessment of planning applications under this policy and represent nationally endorsed best practice. 7.532 Applicants should also refer to Buckinghamshire Council’s SuDS Guidance, which aligns with the National Policy and Standards and provides local interpretation and implementation advice. Examples of SuDS are also set out in the Strategic Flood Risk Assessment. 7.533 SuDS should be designed as an integral part of a hard and soft landscaping scheme to avoid a conflict between a sustainable drainage system and good placemaking. Where SuDs involve natural flood management they should demonstrate they have considered the opportunities for natural flood management in the Local Nature Recovery Strategy. They should also ensure that they do not reduce usable green space. SuDS considerations 7.534 A detailed assessment of site constraints (as set out in CIRIA27) must be undertaken at the outset of the planning process to determine the suitability of SuDS features. Site layouts must be informed by this assessment at an early stage to ensure that sufficient space is provided for above-ground multifunctional SuDS components. Drainage information must be submitted at the application stage. 27 Enabling development. Getting SuDS right from the start (C823F) 7.535 While the National SuDS Standards are currently non-statutory, they represent the most upto-date and nationally endorsed technical guidance on sustainable drainage. As such, they will be treated as a material consideration in the assessment of planning applications under Policy CC2. Applicants should also refer to Buckinghamshire Council’s SuDS Guidance, which aligns with the National Standards and provides local interpretation and implementation advice. 7.536 The National SuDS Standards (2025) were developed collaboratively by DEFRA, the Environment Agency, and industry experts. They reflect the latest understanding of climate resilience, water quality, and multifunctional SuDS design. Their use ensures that developments meet the highest standards of sustainable water management. 7.537 SuDS strategies must be informed by site-specific ground investigations, including infiltration rate testing. Where sites are at risk of high groundwater, ground-water level monitoring over the winter period may be requested to ensure groundwater does not ingress into infiltration components and groundwater is not contaminated through infiltration (through the provision of a suitable freeboard). 7.538 Exemplar source control SuDS should be prioritised. This may include the use of permeable paving for hardstanding areas such as access roads, parking bays and driveways, and patios. The use of SuDS such as basins, ponds, swales, rain gardens/planters and tree pits are also strongly encouraged. Where infiltration is viable, these features can be used to provide storage prior to infiltrating into the underlying geology. Where infiltration is not viable, above-ground SuDS must be prioritised for attenuation and conveyance prior to discharge offsite. 7.539 Brownfield sites typically have higher existing runoff rates compared to greenfield sites due to their existing impermeable area; therefore, the proposed discharge rate may be higher (the proposed discharge rate must not exceed the brownfield rate). However, using well-designed SuDS strategies, it is possible to reduce discharge rates close to the greenfield rate. This approach must be adopted wherever feasible. 7.540 Where the final discharge point is the public sewerage network, the runoff rate agreed by the LLFA will also need to be agreed with the sewerage undertaker (through the provision of a Pre-Planning Enquiry). 7.541 It must be demonstrated that the designed SuDS strategy does not flood up to the 1 in 30year storm event, and any flooding up to the 1 in 100-year storm event plus climate change must be safely contained onsite. Where appropriate, 10% urban creep allowance must be included. 7.542 Green above ground SuDS components are encouraged as they are easier to maintain than below ground SuDS components such as attenuation tank. A maintenance schedule is required to set out who will maintain the system, how the maintenance will be funded and should be supported by an appropriately detailed maintenance and operation manual. 7.543 The council supports making existing hard surface areas to be permeable under this policy. Areas for parking and servicing shall be planned for in a comprehensive manner as part of surface water management. The policy criteria include detailed requirements to address contamination risks. 7.544 Planning applications must demonstrate how the proposed SuDS strategy complies with the National SuDS Standards through submission of a compliance checklist or equivalent summary. More detailed information on how to demonstrate compliance with SuDS requirements can be found in Buckinghamshire Council’s guidance on the website.